The terms “first connection” and “second connection” as used here refer to electrically conductive connections. Through these connections, the energy stored in the energy storage unit can be tapped via the poles of the energy storage unit and made available to the direct voltage circuit at the potential taps in the form of a direct voltage.
Emergency power supply devices for supplying emergency power to a direct voltage circuit find widespread use in the state of the art. A frequent application case is the supply of emergency power to the intermediate circuit of a converter. As a rule, such converters are supplied with three-phase alternating current from an external power network and they supply a consumer, for instance, a motor, with alternating current or direct current. When the three-phase alternating current provided by the external power network is converted, first of all, the three-phase alternating current is rectified. The direct current generated by the rectification is applied to an internal direct current circuit in the inverter. This direct current circuit is referred to as an intermediate circuit. The direct voltage present in the direct current circuit can be tapped via a first potential tap and via a second potential tap. An alternating current for supplying a consumer is generated from the direct voltage that is present in the intermediate circuit, for example, by means of inversion. In the case of safety-relevant consumers, the consumer continues to operate, at least for a certain period of time, even if the external power network fails. Examples of safety-relevant consumers include the motors employed in passenger elevators or the motors of the pitch system of wind turbines.
If the external power network fails, which can happen, for example, in case of a fire in a building, the elevators travel to the next floor and open the doors without the power supplied by the external power network, so that the persons in the elevator can be brought to safety. Conventional drive motors for elevators are supplied with power by means of converters. In order for elevators to be able to perform the above-mentioned emergency procedure in case of a power failure, they are normally equipped with an emergency power supply device that can supply the converter with power in case of an emergency.
Modern wind turbines are usually equipped with electric pitch systems that have a motor as well as a converter to supply power to the motor for each rotor blade. As a rotor blade rotates around its longitudinal axis, such pitch systems regulate the position of the rotor blades relative to the wind, and they are often the only safe way to bring the rotor of a wind turbine to a standstill. This is done in that the pitch system turns the rotor blades into a feathering position, bringing the rotor to a standstill because it is no longer driven by the wind. The pitch system is normally supplied with power by the power network into which the wind turbine also feeds the power it generates. If the power network fails, a hazardous situation can arise, for example, if the rotational speed of the rotor of the wind turbine exceeds the permissible maximum value if the wind picks up, so that the wind turbine could sustain damage or persons present in the vicinity could suffer injury as a result.
In order to avert such a hazardous situation, even if the power network fails, the rotor blades may be moved into the feathering position, even when the pitch system is not being supplied with power by the external power network. For this purpose, it is known from the state of the art to equip the pitch system with an emergency power supply device that, in case of a power network failure, ensures the supply of power to the pitch system and thus the functionality of the pitch system, at least until the rotor blades have been moved into the safe feathering position. With a direct-current motor, the emergency power supply device can also be connected directly to the direct-current motor in case of an emergency.
However, a safety problem arises when an emergency power supply device is connected to the intermediate circuit of a converter. Batteries are often employed as the emergency power supply device. Since the intermediate circuit can have a variable voltage that can also be far higher than the voltage provided by the battery, the batteries usually are not connected directly to the intermediate circuit in order to avoid overcharging of the batteries. In order to prevent a flow of charging current from the intermediate circuit into the batteries, one or more decoupling components are installed in the connection lines between the poles of the battery and the potential taps of the intermediate circuit. Therefore, the task of the decoupling components is to prevent a current flow that corresponds to a charging of the batteries, that is to say, a flow of charging current, but at the same time, to allow a flow of current that corresponds to a discharging of the batteries, that is to say, a flow of discharging current. A flow of discharging current corresponds to precisely the proper use of the batteries, namely, the supply of emergency power to the direct current circuit in case of an emergency. As a rule, diodes or diodes connected in series are used for decoupling purposes and, in view of their function as a decoupling component, they are also referred to as decoupling diodes. The diodes employed may be power diodes, which are used for high voltages and currents. An alternative to the utilization of batteries as the emergency power supply device is the use of capacitors. However, like batteries, capacitors can also be damaged by excessively high voltages provided by the intermediate circuit.
Component defects, ageing processes or overloading, for example, can cause the employed decoupling components to fail. Extraordinarily high voltages in the reverse direction of the decoupling components are a major cause of such a possible failure of the decoupling components. A failure can especially consist of a breakdown in an employed decoupling diode. Such a breakdown causes the decoupling diode to become conductive in the reverse direction, so that it is no longer capable of preventing a flow of charging current. If the failure of the decoupling components results in an uncontrolled flow of charging current, the batteries can become overcharged. Outgassing of hydrogen from the batteries can occur if the batteries are overcharged. This constitutes a major safety hazard since the hydrogen gas can form an explosive mixture with the ambient atmosphere, which can easily be ignited by the sources of ignition commonly found in wind turbines. If the batteries are located in a closed container, the pressure build-up inside the container brought about by the outgassing alone can cause the container to rupture, which can lead to considerable damage inside the wind turbine, also due to the subsequent ignition of the hydrogen gas that has escaped from the ruptured container. In the case of very high pressures, even self-ignition can occur.